A conventional wireless local area network (WLAN) can include a plurality of access points or access ports or APs. These APs can be fixed at known locations. An AP allows wireless communication devices (WCDs) associated with that AP to wirelessly connect to a wired network. Each AP connects to a wired network either directly or indirectly via a wireless switch, and can relay data between the WCD and wired computing devices operating as part of the network.
To determine the location of WCDs that are operating in a WLAN, trilateration techniques have been developed. For instance, if a WCD is known to be within communication range of three or more APs, then the estimated location of the WCD can be determined by collecting Received Signal Strength Indicator (RSSI) information at three or more APs, and then using that RSSI information to estimate the distance between the WCD and each of the APs. Once the estimated distances between the WCD and each of the APs are known they can be used along with the coordinates of the APs to determine the estimated location of the WCD.
One technique for collecting RSSI information involves time synchronizing all APs, then causing the APs to simultaneously tune to the same pre-agreed channel at a pre-agreed interval. Each AP then detects a frame sent from the WCD to the APs. One drawback to this approach is that synchronizing multiple access points to switch to a channel at a pre-agreed time is difficult to manage. Another problem is that when its time to locate the WCD, all other APs (except for the one AP that is currently associated with the WCD) are forced to switch to a different channel (i.e., the channel the WCD and its associated AP are communicating on). As such, those APs are forced to drop whatever they are doing (e.g., supporting voice calls on a different channel, etc) since they need to switch “off channel.” This is disruptive to the other WCDs communicating via those APs and does not allow the APs to accommodate local communication conditions by switching to the pre-agreed channel at different times. Moreover, this approach does not scale well to large sites with many APs since synchronization task becomes more difficult.
Another technique for collecting RSSI information involves having the WCD send out a frame (e.g., an IEEE 802.11 probe request frame) on all supported channels within a very short interval of time so that the WCD is “heard” by all nearby APs. Each AP can then measure RSSI information when the frame is detected. A drawback of this approach is that WCDs will only probe when they need information on the network and/or are about to roam. A WCD that happens to be in good communication range of an AP, does not probe very often. Moreover, for the WCD, probing all channels is disruptive for data transfer on the current channel.
Still another technique for collecting RSSI information involves having APs attempt to detect frames from the WCD that is to be located by periodically scanning all channels other than the one they are currently receiving over, and measuring RSSI information if a frame is detected. Such off-channel scanning techniques also have drawbacks. For instance, because there are a large number of channels to be scanned, the duration of each scan (i.e., amount of time spent per channel) and the frequency of each scan (i.e., periodicity with which a channel is scanned) are low. Moreover, the probability of detecting a frame from a WCD during a scan on a particular channel depends greatly on the WCD's activity level (i.e., probability drops significantly if the particular WCD is not particularly active). Thus, detecting a frame from a particular WCD on one of these scans is an inherently “low-probability” endeavor.
Thus, it would be desirable to provide improved techniques for collecting RSSI information at APs in a network so that the RSSI information can be used to locate a WCD operating among APs within the network.